Refrigeration systems are often operated by adjusting a degree of opening of an expansion valve and/or by opening/closing an expansion valve, thereby controlling the amount of liquid refrigerant being supplied to an evaporator. It is desirable to control the expansion valve in such a manner that it is obtained that all of the liquid refrigerant which is supplied to the evaporator is evaporated before exiting the evaporator, and in such a manner that mixed phase refrigerant is present at or immediately before the outlet of the evaporator. In the case that liquid refrigerant is allowed to exit the evaporator there is a risk that this liquid refrigerant reaches the compressor, and this may in some cases cause damage to the compressor. On the other hand, in the case that the liquid refrigerant evaporates while passing the first part of the evaporator, then the refrigeration capacity of the evaporator is not utilized to the full extent.
The superheat of the refrigerant provides information as to whether or not the situation described above has been obtained. Superheat is normally defined as the difference between the actual temperature of a fluid and the boiling point of the fluid. Accordingly, the superheat depends on the temperature as well as the pressure of the fluid. Thus, the superheat is a suitable parameter for controlling the opening degree of the expansion valve. It is normally desired that the refrigerant has a low, but positive, superheat. When this is the case, the situation described above is obtained, i.e. the refrigeration capacity of the evaporator is utilized to the greatest possible extent, and the risk of causing damage to the compressor due to liquid refrigerant being passed through the evaporator is minimised.
In order to be able to control the expansion valve in a manner which ensures that an optimum superheat of the refrigerant is maintained, it is necessary to be able to obtain an accurate measure for the superheat value. To this end it is necessary to calibrate the sensor or sensors used for measuring the superheat, or parameters used for calculating the superheat. This calibration should preferably be very accurate.
It has previously been attempted to obtain an accurate calibration of superheat sensors by performing an individual calibration of each superheat sensor at the factory manufacturing the sensors. This is cumbersome and increases the manufacturing costs. Furthermore, it is crucial that it is ensured that the calibration information is coupled to and follows a correct sensor. In some cases this has been done by storing the calibration information directly on the sensor, e.g. on a circuit board attached to the sensor. This increases the component count for the sensor, and it is cumbersome. In other cases the calibration information has been stored separately, e.g. in a corresponding controller. However, this introduces a considerate risk of introducing errors originating from a mismatch between a sensor and calibration information.
U.S. Pat. No. 5,820,262 discloses a refrigerant sensor that provides within a common assembly pressure temperature and superheat measurements and calculations. The refrigerant sensor includes a pressure transducer for measuring the pressure of the refrigerant material and a temperature transducer for measuring the temperature of the refrigerant material. The pressure and temperature measurements are used by a microprocessor to calculate the superheat value of the refrigerant material. The refrigerant sensor may contain capability for self-calibration. The calibrator calibrates the measurements of the pressure transducer and the temperature transducer based upon data contained within a pressure-temperature calibration data table. Within the pressure-temperature calibration data table, are pressure and temperature values which serve as checks for the measured values done by the pressure transducer and the temperature transducer. The pressure-temperature calibration data table contains a column of pressure values cross-correlated with a column of temperature values for performing the aforementioned calibration.